Electrophotography is a kind of image-forming process, which generally comprises charging the surface of a photoreceptor containing a photoconductive material in the dark by means of, e.g., corona discharge, image-wise exposing the resulting photoreceptor to selectively eliminate the charge in the exposed area to thereby obtain an electrostatic latent image, converting the latent image into a visible image using a toner, transferring the toner image to paper, etc., and then fixing the toner particles to obtain an image.
Photoreceptors include inorganic photoreceptors containing an inorganic photoconductive compound, e.g., selenium, zinc oxide, cadmium sulfide, or silicon, as a major component and organic photoreceptors containing an organic charge generation material and a low- or high-molecular organic charge transporting material both of which are dispersed in a binder resin. The inorganic photoreceptors each has many advantages and has been widely used so far. However, those inorganic photoconductive compounds have the following drawbacks. For example, selenium not only is costly because of the difficulty of the production thereof, but also tends to crystallize and to be readily affected by heat or mechanical shock to thereby suffer performance deterioration. Zinc oxide and cadmium sulfide are insufficient in moisture resistance and mechanical strength, and a dye added as a sensitizer is deteriorated by the charging and exposure. Thus, photoreceptors containing zinc oxide or cadmium sulfide are defective in durability, etc. Silicon is also costly because of the difficulty of the production thereof and because a highly irritant gas is used for producing the same. Moreover, care should be taken in handling silicon because it is sensitive to moisture.
For the purpose of overcoming the drawbacks of these inorganic photoreceptors, organic photoreceptors containing various organic compounds have been investigated in recent years and have come to be used widely. The organic photoreceptors include single-layer photoreceptors in which both a charge generation material and a charge transporting material are dispersed in a binder resin and double-layered photoreceptors which comprise a charge generation layer and a charge transporting layer which layers perform their respective functions. Organic photoreceptors of the double-layer type are advantageous in that each material can be selected from a wide range of compounds and a photoreceptor having a desired performance can be produced relatively easily by selecting a suitable material combination. Because of this, a large number of investigations have been made on double-layered organic photoreceptors, which are in wide use.
As the charge generation materials, various kinds of organic pigments and dyes have been proposed and put to practical use. Examples thereof include azo compounds, bisazo compounds, trisazo compounds, tetrakisazo compounds, thiapyrylium salts, squarilium salts, azulenium salts, cyanine dyes, perylene compounds, metal-free or metal phthalocyanine compounds, polynuclear quinone compounds, thioindigo compounds, and quinacridone compounds.
Examples of charge transporting materials include the oxadiazole compounds disclosed in JP-B-34-5466, the oxazole compounds disclosed in JP-A-56-123544, pyrazoline compounds disclosed in JP-B-52-41880, the hydrazone compounds disclosed in JP-B-55-42380, JP-B-61-40104, JP-B-62-35673, and JP-B-63-35976, the diamine compounds disclosed in JP-B-58-32372, stilbene compounds disclosed in JP-B-63-18738, JP-B-63-19867, and JP-B-3-39306, and the butadiene compounds disclosed in JP-A-62-30255. (The terms "JP-B" and "JP-A" as used herein mean an "examined Japanese patent publication" and an "unexamined published Japanese patent application," respectively.) Some of the organic photoreceptors containing these charge transporting materials have excellent properties and have come into practical use. However, any organic photoreceptor has not been obtained so far which fully satisfies the various property requirements which an electrophotographic photoreceptor is required to meet.
On the other hand, electroluminescent elements containing an organic compound as a component thereof have conventionally been investigated, but sufficient luminescent properties have not been obtained. In recent years, however, a multilayered electroluminescent element comprising several kinds of superposed organic materials was found to show significantly improved properties. Since then, investigations on electroluminescent elements containing organic substances have been made actively. The first multilayered electroluminescent element was reported by C. W. Tang et al. of Eastman Kodak Corp. (Appl. Phys. Lett. Vol. 51, p. 913 (1987)). In this electroluminescent element, a luminance of 1,000 cd/m.sup.2 or higher was obtained at a voltage of 10 V or lower, showing that this organic electroluminescent element has far higher electroluminescent properties than the conventional inorganic electroluminescent elements in practical use, which inorganic elements need a voltage as high as 200 V or higher.
Such multilayered electroluminescent elements have a structure comprising superposed layers of an organic fluorescent substance, an organic substance capable of transporting charges, i.e., a charge transporting material, and electrodes. When charges (holes and electrons) are injected from each electrode, the charges move through the charge transporting material and recombine to cause luminescence. Used as the organic fluorescent substance are, for example, fluorescent organic dyes such as 8-quinolinol aluminum complex and coumarin. For use as the charge transporting material, various compounds well known as organic materials for use in electrophotographic photoreceptors are being investigated. Examples of such compounds for use as charge transporting materials include diamine compounds such as N,N'-di(3-tolyl)-N,N'-diphenyl-4,4'-diaminodiphenyl and 1,1-bis[N,N-di(4-tolyl)amino-phenyl]cyclohexane and hydrazone compounds such as 4-diphenyl-aminobenzaldehyde-N,N-diphenylhydrazone. Also used are porphyrin compounds such as copper phthalocyanine.
Although organic electroluminescent elements have high luminescent properties, they are insufficient in stability during luminescence and in storage stability and have hence not been put to practical use. As one of the causes of the insufficient stability during luminescence and storage of the element, the insufficient stability of the charge transporting material is considered. Since the organic layers of an organic electroluminescent element are as thin as from 50 to hundreds of nanometers, an exceedingly high voltage is applied per unit thickness. In addition, heat generation occurs due to luminescence and the application of an electric current. The charge transporting material is therefore required to have electrical, thermal, and chemical stability. Furthermore, the charge transporting layer of an element, which layer is generally in an amorphous state, undergoes crystallization due to luminescence or long-term storage, whereby luminescence is inhibited or element breakage is caused. For this reason, the charge transporting material is required to have the property of readily coming into an amorphous or vitreous state and stably retaining this state.
With respect to the compounds proposed for use as a charge transporting material, by which the stability of a luminescent element is affected as described above, the diamine compounds and the porphyrin compounds still have the problem that they undergo crystallization to cause element deterioration, although these compounds are electrically and thermally stable and some of these attain relatively high luminescent properties. The hydrazone compounds, which have a simple structure, are insufficient in electrical and thermal stability, so that they are not a desirable material.
A charge transporting material for use in an organic photoreceptor is required not only to enable the photoreceptor to satisfy various property requirements including sensitivity, but also to have chemical stability so as to withstand light, ozone, and electrical load and further have stability or durability so as not to cause a sensitivity decrease even when the photoreceptor is used repeatedly or over long.
A charge transporting material for use in an organic electroluminescent element is required not only to enable the element to satisfy various property requirements including luminescent properties, but also have good film-forming properties so as to impart excellent stability during luminescence and storage to the organic electroluminescent element and also have stability so as to withstand heat, oxygen, and electrical load.